feat: ADR-0035 4i(a,b) — CREATE TABLE help/usage + describe table constraints; Phase 4 complete

(b) describe shows table-level constraints: TableDescription gains
unique_constraints + check_constraints (populated by do_describe_table
from read_schema), rendered in a new "Table constraints:" section —
composite UNIQUE and table-level CHECK (named + unnamed). The per-column
Constraints column already covered single-column NOT NULL/UNIQUE/PK/CHECK.

(a) CREATE TABLE help/usage skeleton refreshed for the column DEFAULT/
CHECK/REFERENCES, table-level composite UNIQUE, table CHECK, and
table-level FOREIGN KEY forms (4a.2/4a.3/4b) — engine-neutral,
vocab-audit clean.

With 4i's (c)/(d)/(e) already shipped, this completes sub-phase 4i — the
verification sweep — and therefore ADR-0035 Phase 4 (4a–4i). ADR-0035
Status, §13 4i, the ADR index, and requirements.md Q1 updated to
"Phase 4 complete".

Tests: render_structure table-level-constraints unit test +
e2e_describe_shows_table_level_constraints. Full suite 1917 passing /
0 failing / 1 ignored; clippy clean.

docs/requirements.md

@@ -256,8 +256,10 @@ handoff-14 cleanup; 449 after B2/C2.)
   CHECK text that qualifies a column with the old table name** so a fresh
   rebuild round-trips; refuses same-name / existing-target / `__rdbms_*` /
   non-existent; auto-named indexes + relationships kept stale per §6
-  scope; one undo step). Remaining: the 4i verification sweep per
-  ADR-0035 §13.)*
+  scope; one undo step), then the **4i verification sweep** (shared-entry-
+  word completion merge + simple/advanced completion colour; `describe` of
+  table-level constraints; self-ref FK pre-submit indicator; CREATE-TABLE
+  help/usage refresh). **ADR-0035 Phase 4 (4a–4i) is complete.**)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
   *(Design done — ADR-0030 §8: out-of-subset statements are